Update aggregation.asciidoc (#24042)

There are four kinds of aggregations now, not three.

docs/reference/aggregations.asciidoc

@@ -11,7 +11,7 @@ the execution defines what this document set is (e.g. a top-level aggregation ex
 query/filters of the search request).
 
 There are many different types of aggregations, each with its own purpose and output. To better understand these types,
-it is often easier to break them into three main families:
+it is often easier to break them into four main families:
 
 <<search-aggregations-bucket, _Bucketing_>>::
 				A family of aggregations that build buckets, where each bucket is associated with a _key_ and a document